<html>
<meta charset="utf-8">

<head>
    <link rel="stylesheet" type="text/css" href="mySheet.css">
    <script src="jquery-3.2.1.min.js"></script>
    <style>
        table tr.a td.a {
            border: 1px solid gray;
        }

        table.no,
        tr,
        td {
            border-collapse: collapse;
            border: 1px solid gray;
        }
    </style>
</head>
<div>
    <ul>
        <li>设置表格边框border: 大小 画笔 颜色 border:1px solid black</li>
        <li>折叠边距 border-collapse:collapse</li>
    </ul>
    <h3>只设置边框的表格 因为tr和td都有边距线，所以是双线</h3>
    <table>
        <tr class="a">
            <td class="a">123</td>
            <td class="a">345</td>
            <td class="a">321</td>
        </tr>
        <tr class="a">
            <td class="a">asx</td>
            <td class="a">ccc</td>
            <td class="a">ddd</td>
        </tr>
        <tr class="a">
            <td class="a">eee</td>
            <td class="a">fff</td>
            <td class="a">ggg</td>
        </tr>
    </table>
    <h3>折叠边距</h3>
    <table class="no">
        <tr>
            <td>2222</td>
            <td>3333</td>
        </tr>
        <tr>
            <td>4444</td>
            <td>5555</td>
        </tr>
    </table>
</div>

</html>